This invention relates to apparatus for generating constant-envelope, angle-modulated signals, such as a sinusoidal frequency shift keyed (SFSK) signal. Such signals are useful in connection with modern digital communications systems for transmitting data or other signals using digital modulation. In particular the SFSK modulation technique is a quadrature phase modulation technique which has essentially a uniform amplitude signal envelope. The uniform amplitude signal envelope arises out of a sinusoidal shift between the inphase-antiphase portion of a quadrature phase signal and the quadrature portion of the signal.
SFSK signals are conventionally developed using apparatus which provides two specially shaped pulsed modulating signals which are used to amplitude modulate quadrature phase carrier signals with shaped interleaved pulses. After modulation, the carriers are combined to provide the composite four phase SFSK signal.
It is an object of the present invention to provide a new, improved, and simplified modulator for generating a constant-envelope, angle-modulated pulse signal using an acoustic surface wave filter.